In certain technological processes and fabrication procedures, as well as in many every-day situations, it is of crucial importance to utilize objects with strongly water repellent and/or self cleaning surfaces that are stable enough to retain the water-repellent and/or self cleaning property even after water exposure.
The water repellant ability of a surface is positively correlated to the hydrophobicity and the degree of unevenness on a colloidal length scale, i.e. about 10−8-10−5 m, thereof. A highly water repellant surface is resulting when the actual contact surface with water, on the microscopic scale, is much larger than the projected macroscopic surface. Hydrophobic surfaces with a suitable microscopic geometry may be highly water repellant as indicated by a contact angle toward pure water in the range between about 150 and 180 degrees. Such surfaces are commonly denoted superhydrophobic surfaces. A well-known example taken from nature itself is the leaf of the lotus plant (Nelumbo nucifera). A superhydrophobic surface is, as a rule, “self-cleaning” which means that particles of dust and dirt which at first adhere to the surface are being transferred to water droplets sprinkled onto the surface and then removed when the droplets roll off the surface.
Much effort has been devoted to the application of waxes and similar hydrophobic materials, e.g. alkylketenedimer (AKD), onto various surfaces for rendering these water repellant and, occasionally, even superhydrophobic.
Torkkeli et al., Micro Electro Mechanical Systems, 2001, pp 475-478, describes a method for rendering the surface of an electric insulator superhydrophobic by spraying a chloroform solution of AKD onto the same. Disadvantages of this method include a low reproducibility and a high toxicity of the chloroform employed. Chloroform is e.g. generally considered as carcinogenic and is associated with a negative environmental impact. Furthermore, the use of chloroform as solvent limits the spraying device employed to only such spraying devices in which contacting parts, e.g. gaskets and other polymeric materials, are not unacceptably affected by chloroform, which is a solvent with a high dissolution capability for many polymeric materials and/or which may cause such to swell.
WO2009005465A1 describes a method for the preparation of a superhydrophobic surface by expansion of a pressurized fluid in which a hydrophobic substance is dissolved. The substance crystallize/precipitate after expansion whereby particles are formed and deposited to form a superhydrophobic surface. Disadvantages of this method include the need of equipment capable of storing and handling pressurized solutions. Furthermore, the solubility of the hydrophobic substance in the pressurized solvent, typically supercritical CO2, is relatively low. Hence, a relatively large volume of pressurized solvent is needed for the deposition of a relatively small amount of hydrophobic substance.
Hence, improved methods for the production of superhydrophobic surfaces are highly desired.